Definition of Triplet

Triplet: One of three, as:
1. One of three babies in a multiple birth. Or three animals gestated together.
2. In genetics, a set of three bases in DNA. Also called a codon. See: Crick-Brenner experiment.
3. In optics, a combination of three simple lenses cemented together.

The word triplet came into use in the mid-17th century. It was formed from triple, just as doublet was formed from double.